Internet DRAFT - draft-ietf-core-multipart-ct
draft-ietf-core-multipart-ct
CoRE T. Fossati
Internet-Draft ARM
Intended status: Standards Track K. Hartke
Expires: February 22, 2020 Ericsson
C. Bormann
Universitaet Bremen TZI
August 21, 2019
Multipart Content-Format for CoAP
draft-ietf-core-multipart-ct-04
Abstract
This memo defines application/multipart-core, an application-
independent media-type that can be used to combine representations of
zero or more different media types into a single message, such as a
CoAP request or response body, with minimal framing overhead, each
along with a CoAP Content-Format identifier.
Status of This Memo
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Copyright Notice
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Multipart Content-Format Encoding . . . . . . . . . . . . . . 4
3. Usage Example: Observing Resources . . . . . . . . . . . . . 4
4. Implementation Hints . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5.1. Registration of media type application/multipart-core . . 6
5.2. Registration of a Content-Format identifier for
application/multipart-core . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
1. Introduction
This memo defines application/multipart-core, an application-
independent media-type that can be used to combine representations of
zero or more different media types, each along with a CoAP Content-
Format identifier, into a single representation, with minimal framing
overhead. This combined representation may then be carried in a
single message, such as a CoAP [RFC7252] request or response body.
This simple and efficient binary framing mechanism can be employed to
create application specific request and response bodies which build
on multiple already existing media types.
As the name of the media-type suggests, it is inspired by the
multipart media types that started to be defined with the original
set of MIME specifications [RFC2046]. However, while those needed to
focus on the syntactic aspects of integrating multiple
representations into one e-mail, transfer protocols providing full
data transparency such as CoAP as well as readily available encoding
formats such as the Concise Binary Object Representation (CBOR)
[RFC7049] shift the focus towards the intended use of the combined
representations. In this respect, the basic intent of the
application/multipart-core media type is like that of multipart/mixed
(Section 5.1.3 of [RFC2046]). The detailed semantics of the
representations are refined by the context established by the
application in the accompanying request parameters, e.g., the
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resource URI and any further options (header fields), but three usage
scenarios are envisioned:
The individual representations in an application/multipart-core body
occur in a sequence, which may be employed by an application where
such a sequence is natural, e.g. for a number of audio snippets in
various formats to be played out in that sequence, or search results
returned in order of relevance.
In other cases, an application may be more interested in a bag of
representations, which are distinguished by their Content-Format
identifier, such as an audio snippet and a text representation
accompanying it. In such a case, the sequence in which these occur
may not be relevant to the application. This specification adds the
option of substituting a null value for the representation of an
optional part, which indicates that the part is not present.
A third situation that is common only ever has a single
representation in the sequence, where the sender already selects just
one of a set of formats possible for this situation. This kind of
union "type" of formats may also make the presence of the actual
representation optional, the omission of which leads to a zero-length
array.
Where these rules are not sufficient for an application, it might
still use the general format defined here, but register a new media
type and an associated Content-Format identifier to associate the
representation with these more specific semantics instead of using
the application/multipart-core media type.
Also, future specifications might want to define rough equivalents
for other multipart media types with specific semantics not covered
by the present specification, such as multipart/alternative
(Section 5.1.4 of [RFC2046]), where several alternative
representations are provided in the message, but only one of those is
to be selected by the recipient for its use (this is less likely to
be useful in a constrained environment that has facilities for pre-
flight discovery).
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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2. Multipart Content-Format Encoding
A representation of media-type application/multipart-core contains a
collection of zero or more representations, each along with their
respective content format.
The collection is encoded as a CBOR [RFC7049] array with an even
number of elements. Counting from zero, the odd-numbered elements
are a byte string containing a representation, or the value "null" if
an optional part is indicated as not given. The (even-numbered)
element preceding each of these is an unsigned integer specifying the
content format ID of the representation following it.
For example, a collection containing two representations, one with
content format ID 42 and one with content format ID 0, looks like
this in CBOR diagnostic notation:
[42, h'0123456789abcdef', 0, h'3031323334']
For illustration, the structure of an application/multipart-core
representation can be described by the CDDL [RFC8610] specification
in Figure 1:
multipart-core = [* multipart-part]
multipart-part = (type: uint .size 2, part: bytes / null)
Figure 1: CDDL for application/multipart-core
This format is intended as a strict specification: An implementation
MUST stop processing the representation if there is a CBOR well-
formedness error, a deviation from the structure defined above, or
any residual data left after processing the CBOR data item. (This
generally means the representation is not processed at all except if
some streaming processing has already happened.)
3. Usage Example: Observing Resources
This section illustrates one less obvious example for using
application/multipart-core: combining it with observing a resource
[RFC7641] to handle pending results.
When a client registers to observe a resource for which no
representation is available yet, the server may send one or more 2.05
(Content) notifications before sending the first actual 2.05
(Content) or 2.03 (Valid) notification. A diagram depicting possible
resulting sequences of notifications, identified by their respective
response code, is shown in Figure 2.
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__________ __________ __________
| | | | | |
---->| 2.05 |---->| 2.05 / |---->| 4.xx / |
| Pending | | 2.03 | | 5.xx |
|__________| |__________| |__________|
^ \ \ ^ \ ^
\__/ \ \___/ /
\_______________________/
Figure 2: Sequence of Notifications
The specification of the Observe option requires that all
notifications carry the same Content-Format. The application/
multipart-core media type can be used to provide that Content-Format:
e.g., carrying an empty list of representations in the case marked as
"Pending" in Figure 2, and carrying a single representation specified
as the target content-format in the case in the middle of the figure.
4. Implementation Hints
This section describes the serialization for readers that may be new
to CBOR. It does not contain any new information.
An application/multipart-core representation carrying no
representations is represented by an empty CBOR array, which is
serialized as a single byte with the value 0x80.
An application/multipart-core representation carrying a single
representation is represented by a two-element CBOR array, which is
serialized as 0x82 followed by the two elements. The first element
is an unsigned integer for the Content-Format value, which is
represented as described in Table 1. The second element is the
object as a byte string, which is represented as a length as
described in Table 2 followed by the bytes of the object.
+----------------+------------+
| Serialization | Value |
+----------------+------------+
| 0x00..0x17 | 0..23 |
| 0x18 0xnn | 24..255 |
| 0x19 0xnn 0xnn | 256..65535 |
+----------------+------------+
Table 1: Serialization of content-format
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+-----------------------------+-------------------+
| Serialization | Length |
+-----------------------------+-------------------+
| 0x40..0x57 | 0..23 |
| 0x58 0xnn | 24..255 |
| 0x59 0xnn 0xnn | 256..65535 |
| 0x5a 0xnn 0xnn 0xnn 0xnn | 65536..4294967295 |
| 0x5b 0xnn .. 0xnn (8 bytes) | 4294967296.. |
+-----------------------------+-------------------+
Table 2: Serialization of object length
For example, a single text/plain object (content-format 0) of value
"Hello World" (11 characters) would be serialized as
0x82 0x00 0x4b H e l l o 0x20 W o r l d
In effect, the serialization for a single object is done by prefixing
the object with information that there is one object (here: 0x82),
about its content-format (here: 0x00) and its length (here: 0x4b).
For more than one representation included in an application/
multipart-core representation, the head of the CBOR array is adjusted
(0x84 for two representations, 0x86 for three, ...) and the sequences
of content-format and embedded representations follow.
For instance, the example from Section 2 would be serialized as:
0x84 (*) 0x182A 0x48 0x0123456789ABCDEF (+) 0x00 0x45 0x3031323334
where (*) marks the start of the information about the first
representation (content-format 42, byte string length 8) and, (+), of
the second representation (content-format 0, byte string length 5).
5. IANA Considerations
5.1. Registration of media type application/multipart-core
IANA is requested to register the following media type [RFC6838]:
Type name: application
Subtype name: multipart-core
Required parameters: N/A
Optional parameters: N/A
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Encoding considerations: binary
Security considerations: See the Security Considerations Section of
RFCthis
Interoperability considerations: N/A
Published specification: RFCthis
Applications that use this media type: Applications that need to
combine representations of zero or more different media types into
one, e.g., EST-CoAP [I-D.ietf-ace-coap-est]
Fragment identifier considerations: The syntax and semantics of
fragment identifiers specified for "application/multipart-core" is
as specified for "application/cbor". (At publication of this
document, there is no fragment identification syntax defined for
"application/cbor".)
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person & email address to contact for further information:
iesg&ietf.org
Intended usage: COMMON
Restrictions on usage: N/A
Author: CoRE WG
Change controller: IESG
Provisional registration? (standards tree only): no
5.2. Registration of a Content-Format identifier for application/
multipart-core
IANA is requested to register the following Content-Format to the
"CoAP Content-Formats" subregistry, within the "Constrained RESTful
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Environments (CoRE) Parameters" registry, from the Expert Review
space (0..255):
+----------------------------+----------+------+-----------+
| Media Type | Encoding | ID | Reference |
+----------------------------+----------+------+-----------+
| application/multipart-core | -- | TBD1 | RFCthis |
+----------------------------+----------+------+-----------+
6. Security Considerations
The security considerations of [RFC7049] apply. In particular,
resource exhaustion attacks may employ large values for the byte
string size fields, or deeply nested structures of recursively
embedded application/multipart-core representations.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[I-D.ietf-ace-coap-est]
Stok, P., Kampanakis, P., Richardson, M., and S. Raza,
"EST over secure CoAP (EST-coaps)", draft-ietf-ace-coap-
est-12 (work in progress), June 2019.
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[RFC2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC 2046,
DOI 10.17487/RFC2046, November 1996,
<https://www.rfc-editor.org/info/rfc2046>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
June 2019, <https://www.rfc-editor.org/info/rfc8610>.
Acknowledgements
Most of the text in this draft is from earlier contributions by two
of the authors, Thomas Fossati and Klaus Hartke. The re-mix in this
document is based on the requirements in [I-D.ietf-ace-coap-est],
based on discussions with Michael Richardson, Panos Kampanis and
Peter van der Stok.
Authors' Addresses
Thomas Fossati
ARM
Email: thomas.fossati@arm.com
Klaus Hartke
Ericsson
Torshamnsgatan 23
Stockholm SE-16483
Sweden
Email: klaus.hartke@ericsson.com
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Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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